Positioning motor



Allg' 22, 1961 H. BowDITcH 2,997,637

PosITIoNING MOTOR 8 Y Y 7p l N V EN TOR Hoa Bau/wmf# A1182 22, 1961 H. L. BOWDITCH 2,997,637

POSITIONING MOTOR Filed may 26, 1958 2 Sheets-Sheet 2 @www ATTOR '5" nited bitates iiiatem 2,997,637 PSITINING MTR Hoei Ii. Bowditch, Foxboro9 Mass., assigner to The Foxboro Company, Foxboro, P/iass., a corporation of Massachusetts Filed May 26, i958, Ser. No. 737,664

Claims. (Ci. B18-37) This invention relates to an electric motor for positioning such elements as the pen of a recording instrument, the rebalancing member of an automatic measuring circuit, and so forth.

An object of the invention is to provide an electric positioning motor which is simple and inexpensive to manufacture, does not require critical tolerances, but which is very precise and reliable in operation.

Another object is to provide Such a motor which can be controlled by relatively small amounts of power and which responds to control signals quickly and without overshoot.

These and other objects will in part be understood from and in part pointed out in the description given hereinafter.

Modern industry uses a Wide variety of apparatus to automatically control and record such conditions as temperature, pressure, rate-of-flow, and the like. In Such apparatus, the mechanical re-balancing of a condition sensing element, the closing or opening of a valve, or the movement of a pen across a recording chart are all executed by a positioning motor of one kind or another. To meet the high standards of perfection required in this service, a positioning motor must be free of harmful bearing drag or play, and it must respond easily to relatively weak control signals. It must also be able to quickly reach a position which accurately accords with any given control signal, then stop without overshoot or hunting, and thereafter maintain its position until directed to a new one. Finally, because such a motor is in one sense the heart of the instrument or apparatus and is frequently located in cramped or inaccessible places, it must operate unattended for years with absolute reliability, and yet it must be small in size. These requirements have, in the past, been very diflcult to fulfill and many motors previously developed have been too expensive or else not suiiiciently reliable or precise in operation.

The present invention provides a positioning motor having all the desirable features of simplicity, low cost, reliability and preciseness in operation. In accordance with the invention there is provided a motor which can be energized with standard 60-cycle alternating current and very little power. In the specific motor illustrated herein the movable element, somewhat analogous to the armature in a conventional electric motor, is a thin flexible band stretched around two spaced pulleys to form two parallel sections which are held taut by a spring connecting the ends of the band. Acting on the mid-portion of one section of the band is a clamping and flexing mechanism, energized by simple electromagnets, which by repeatedly bowing a short length of the section serves to inch the band along in steps, in either direction as desired, in much the Way a caterpillar creeps along the ground. Each step taken by the band is small but the steps are taken 60 to the second so that the band can move at considerable speed. The inertia of the band is essentially negligible and since it inherently comes to a complete stop between each step there is no overshoot on reaching a position. When there is no control signal the band is clamped against movement so that the jarring eiects of external vibrations or shocks is minimized. This motor s mechanically easy to construct and its parts are essentially self-aligning.

A better understanding of the invention and a fuller fice 2 appreciation of its many advantages will best be gained from a study of the following description given in connection with the accompanying drawings wherein:

FIGURE l is a perspective view partly broken away of a strip chart recorder using a positioning motor embodying features of the invention;

FIGURE 2 is an enlarged plan view of the motor showing it substantially to scale;

FIGURE 3 is a section view taken as indicated by lines 3 3 in FIGURE 2;

FIGURE 4 is a section view taken as indicated by lines 4 4 in FIGURE 2;

FIGURE 5 is a schematic representation of the motor illustrating the magnetic ux paths;

FIGURES 6(a), (b) and (c) are graphs of the magnetic flux versus time in the clamping and bowing electromagnets of the motor; and

FIGURES 7, 8 and 9 indicate the position (in exaggerated scale) of the motor elements at the Successive times indicated by lines 7 7, 8 3 and 9 9, respectively, in FIGURE 6.

rIhe chart recorder 10 shown in FIGURE l includes a housing 12 having a front window behind which is positioned a chart 14 mounted on two rollers In and 18 and driven slowly downward by a clock motor (not shown). In contact with the upper portion of the chart, and adapted to swing in either direction across it, is an indicator pen 2% which records, in accordance with its lateral position on the chart, a variable condition such as temperature. The rear end of this pen carries a counterweiht 22 which is closely in front of the base plate 24 of an electric motor generally indicated at 26, Just forward of this counter weight the pen is attached at point 28 to the upper section 32 of a band 34.

The band 34 is a flexible ribbon of metal, and is stretched around and between the two idler pulleys 36 and 38. The latter in turn are rotatably mounted on the axles 4@ and 42 which are supported by the brackets 44 and 46 secured to the base plate 24, The band is kept taut by means of a tension spring 48 inserted in the top section 32. As will appear, this band can be driven in either direction and in so moving will carry the indicating pen 2t) transversely across the chart i4. The position of the band, and with it the pen, is precisely controlled in the way explained below.

Band 34 has a lower section Sti (see also FIGURE 2) which is engaged on one side by a clamping and flexing mechanism generally indicated at S2. This mechanism includes the two laterally movable clamping bars 54 and 56, which are permanently magnetized and between which is mounted a laterally movable bowing arm 58 formed of soft magnetic material. Positioned on the opposite side of the lower section Sil of the band are three electromagnets oil, 62 and 64 which cooperate with the clamping bars 4- and 56 and the bowing `arm S8 respectively.

These electromagnets dit, 62, and till are mounted, along with the clamping bars S4, and S6 and the bowing arm 5S, on a support plate 66 formed of soft magnetic material, and which in turn is fastened (see also FIG- URE 3) to the base plate 24. The right-hand electromagnet of?, seen in FIGURE 4, includes a cylindrical magnetic core 68 the bottom of which is anchored in the support plate 66 and the top of which carries a pole piece 70. Surrounding this core is a multi-turn coil 71 which is adapted to be supplied with alternating current to generate a magnetic flux in the core and pole piece. The other electromagnet 62 similarly includes a magnetic core 72 (see FIGURE 5) fastened at one end to the support plate 66, having a pole-piece 74 at the other end, and surrounded by a coil 75. The center electromagnet seen in FIGURE 3, has a magnetic core 78, and a surrounding coil 80. The outer or right end of core 78 is provided with an adjustable pole piece 82 in the form of a screw which can be set to the proper lateral position relative to the band section Sil for the desired amount of bowing of the band. The forward end of this adjustable pole piece is cushioned by a plastic pad 83, of nylon for example, to minimize wear and to provide an air gap for the magnetic ux to and from the bowing arm 58. The outer or right end of the bowing arm carries a circular pole piece which has a peripheral groove 85 into which the band section 50 can lit.

The lower ends of each of the clamping bars 54 and 56, and of bowing arm 53, are hinged to support plate 66 so that their top ends are swingable laterally toward and away from a respective one of pole pieces '70, 74 and 82. As seen in FIGURE 4, clamping bar 54 is hinged to support plate 66 by a thin flat iiexure 86 the lower end of which is bolted to support plate 66 and which along its upper portion is brazed to the inner face of clamping oar 54. Clamping bar 56 is hinged in the same way by a iiexure (not shown). Similarly, as seen in FIGURE 3, the lower end of the bowing arm 53 is joined by a fiexure SS to support plate 66. Each of these hinge flexures is of soft magnetic material.

To illustrate as clearly as possible the several magnetic flux paths in motor 26, there is shown in FIGURE a partly schematic perspective view of the motor taken from its side opposite to that seen in FIGURE l. For the reason which will be explained shortly, the permanent magnets of the clamping bars 54 and 56 have their like poles extending in the same directions. In FIG- URE 5, the movable ends of these bars are shown as north (N) poles and their hinged ends adjacent support plate 66 are south (S) poles. Bowing arm 5S, by virtue of its proximity to clamping bars 54 and 56 is north magnetic at its movable end. Accordingly, each of these magnetized members establishes lines of magnetic ux, as indicated by the three solid arrows 90, 92 and 94 respectively, which extend across from the movable or upper ends of bars 54 and 56 and arm 55 to the corresponding pole pieces 7i), 74 and 82 of the three electromagnets, downward through the cores, and then within plate 66 back to the lower ends of the clamping bars and upward.

When no current is owing in the coils of the electromagnets, `the clamping bars and bowing arm, because of the permanent magnetic ilux, will be drawn against band section 50 and thus hold it against the stationary pole pieces of the electromagnets. On the other hand, when an alternating current is applied to any one of the electromagnets there is generated an alternating magnetic ux, represented in the three electromagnets by the dotted arrows 96, 93 and ltltl, respectively, and the opposing clamping bar or bowing arm will cyclicly be repelled from the electromagnet thus releasing its pressure against band section 50. By operating the clamping and releasing actions of the clamping bars 54 and 56 and bowing arm 58 in proper phases relative to each other, band section 50 can be inched along to either the right or the left.

As can be seen in FIGURE 5, the turns of coils 7l and 75 are wound in the same sense on their respective cores. However, these coils are connected in reverse sense to the supply terminals ltlZ and llld. Thus, the lower end of coil 7i and the upper end of coil 75 are both connected to terminal 1102. The center electromagnet coil is independently connected to the terminals 166 and M3.

To energize the three electromagnets, terminals 102 and 104 can be connected to a llO volt, 60 cycle power source for example, and terminals 106 and 108 connected to a control apparatus (not shown) which, in accordance with the direction in which the band is to be driven, supplies a similar voltage either in phase or out of phase with the source voltage. The alternating magnetic iiux induced in the cores of the two electromagnets 60 and 62 causes clamping bars 5d and 56 to be attracted and repelled alternately, 180 out of phase with each other, toward and away from the opposing pole pieces 76 and 74. Assuming that current is applied to the center coil 80 in phase with the current to coil 75, then bowing arm 53 will move toward and away from pole piece 82 in phase with the movement of clamping bar 56.

FIGURE 6(a) indicates by the sinusoidal curve 120 the magnetic flux generated in the right-hand core 68 by current in coil 7l. Superimposed on this curve is a dotted line E22 which indicates the steady magnetic flux induced in the core by the permanent magnetism of clamping bar 54. When the alternating iiux line l2() exceeds the level of line 122, pole piece 7l) becomes in effect a north pole and thereupon repels the upper end of clamping bar 54. On the other hand, when alternating line l2@ lies below line 122, pole piece 7@ is attracted to clamping bar 54- and section 59 of endless band 34 is clamped between them. By providing a steady flux, the alternating iiux is biased so that clamping bar 54 is repelled from pole piece 7) once each cycle.

The magnetic flux in core 72 is illustrated in FIG- URE 6(b), line 124 indicating the alternating ilux and line 126 indicating the steady ux induced by the permanent magnetism of clamping bar 56. It will be noted that alternating line 124 is 180 out of phase with alternating line in FIGURE 6(a). Finally, FIGURE 6(6) illustrates the ilux in core 78, solid line 128 indicating the alternating ux when coil 80 is energized in phase with coil 75, and dotted line 130 indicating the alternating ux when coil Si) is energized in phase with coil 71. The straight, horizontal dotted line 132 indicates the steady magnetic flux passing from bowing arm 5S to core 78.

FIGURES 7, 8 and 9 illustrate the sequential operation of the clamping bars and bowing arm as they act in inching band section 56 to the left. To give such leftward movement of the band, coils 75 and 80 are connected in phase so that the alternating fluxes induced in pole pieces 74 and 82 are in phase as represented by solid line 124 in FIGURE 6(1)) and by solid line 128 in FIGURE 6(6). At the instant of time represented by section lines 7-7 in FIGURE 6, clamping bars 54 and 56 and bowing arm 55 will occupy the positions shown in FIGURE 7. (The clamping bar 54 is shown a substantial distance away from the band section 5G* only for explanatory purposes; actually the distance of separation normally will be only a few thousandths of an inch.) At this instant the alternating flux in pole piece 70 is inducing a north (N) pole which repels clamping bar 54. while the fluxes in pole piece 74 and pole piece 82 are inducing south (S) poles which attract clamping bar 56 and bowing arm 58, respectively. In this position, band section 50 is bowed, its right end having been drawn to the left by the small amount required in bowing it. A quarter-cycle later, indicated by lines S-S in FIG- URE 6, the parts occupy the position shown in FIGURE 8. Now, band section is held by both clamping bars 56 and 54, the permanent magnetism of the latter two pulling them against pole pieces '76 and 74 which at this instant are not magnetized by alternating flux. Another quarter cycle later, indicated by lines 9--9l in FIGURE 6, and as shown in FIGURE 9, clamping bar 56 and the bowing arm will have moved away from pole pieces 74, 82 which have become in eiect north poles, and the left-hand end of band section Si) will have moved incrementally to the left under the tension of spring 43 and of the straightening force of the band itself, the right-hand end of the band section being held stationary between clamping bar 54 and pole piece 7l?.

The mode of operation continues until section 50 of band 34- reaches the position desired whereupon the current to coil 80 is turned off and the motor stops. It is not necessary to de-energize coils 7l and 75 since they merely actuate the clamping bars, no motion being imparted to the band unless coil Si) is energized. To reverse the direction ot travel of the band, coil 80 is energized in phase with coil 7l.

rl`he above description of the invention is intended in illustration and not in limitation thereof. Various changes may occur to those skilled in the art and these may be made without departing from the spirit or scope of the invention as set forth.

I claim:

l. A motor for positioning instruments and the like, said motor comprising a frame, a pair of spaced apart pulleys supported from said frame, a thin metal ribbon stretched around said pulleys and having a tension spring pulling its two ends together to form an endless band, first, second and third electromagnets spaced along one portion of said ribbon generally midway between said pulleys, each electromagnet having an upstanding core fixed to a magnetic portion of said frame and having a coil surrounding said core, the outer ends of said cores having pole pieces laterally extending toward said ribbon, the pole pieces of the first and third electromagnets being closer to said ribbon than the pole piece of said second electromagnet, rst, second and third upstanding magnetic members hinged to said frame and extending generally parallel to said electromagnets, the outer ends of said members being opposite respective ones of said pole pieces on the opposite side of said ribbon, and means to energize with alternating current said iirst and third electromagnets out or phase to each other and to energize with alternating current said second electromagnet selective in phase with either said first or third electromagnet.

2. The motor as in claim l wherein said first and third members are permanently magnetized, and wherein the pole piece of said second electromagnet is laterally adjustable.

3. A positioning motor for precise control of instruments and the like, said motor comprising a generally straight strip of relatively un-strctchable but laterally deflectable material, two clamping electromagnets and a deflecting electromagnet therebetween mounted on a magnetic base plate with their ends spaced apart along one side of said strip and with said clamping electromagnets closely adjacent said strip and said deliecting electromagnet slightly away from said strip, three movable members of magnetic material on the other side of said strip, each member being opposite a respective one of said electromagnets and laterally movable against and away from said strip, and means to energize said clamping electromagnets out of phase with each other and to energize said deilecting electromagnet selectively in phase with either one of said clamping electromagnets to drive said strip in either direction.

4. A motor for precisely positioning instrument pointers and the like, said motor comprising a frame, a pair of support means mounted on said frame, a thin generally unstretchable strip supported by said support means and movable in either direction, spring means tensioning said strip, first, second and third laterally movu able members spaced along one portion of said strip generally midway between said support means, each member having an end piece laterally extending toward said strip, first and second clamping elements fixed to said frame and being positioned opposite respective ones of said first and third members on the opposite Side of said strip, and means to energize with alternating movement said first and third movable members out of phase to each other and to energize with alternating movement said second movable member selective in phase with either said first or third movable member to drive said strip in either direction.

References Cited in the file of this patent UNITED STATES PATENTS 1,396,337 McDonald Nov. 8, 1921 1,941,597 Cavagnaro Jan. 2, 1934 2,646,518 Thompson July Z1, 1953 

